Computing, communications, consumer electronics and other processor-based systems are driven to host a larger number of applications, each with increasing complexity. The transfer of information and signals required among the components of these processor-based systems leads to increasing performance demands on the circuit devices involved in transfer of data. For example, memory system speeds and capacity may be increased to satisfy a demand for more applications.
An example of a memory type that can be used with these systems is a double-data rate synchronous dynamic random access memory (e.g., the DDR-SDRAM family of products, which includes products of all DDR generations, including DDR1, DDR2, DDR3, DDR4, etc.). The DRAM is considered synchronous as it coordinates its operations to a provided clock signal, and it is considered double-data rate as it transfers data on both the rising and falling edge of the clock.
DDR DRAM devices may transmit and receive data using a strobe-based method. In this method, a strobe signal (which may be referred to as a DQS signal in a DDR system but may be referred to by other acronyms in other types of systems) is edge-aligned to and accompanies a group of data signals (which may be referred to as the DQ signals) sent by the DRAM in a “read” operation. Also in this method, a strobe signal (DQS) is center-aligned (also referred to as “quadrature aligned” since DQS is offset from the data edge by a quarter of the clock cycle time) to and accompanies the data signals (DQ signals) in a “write” operation. The DQS signals are then used by the receiving devices to time the sampling of the data signal. For example, in the memory controller (which is the receiving device during memory read operations), the DQS signal and the data are received and the DQS signal is delayed by some fixed amount, nominally one-fourth of the memory system clock period. This delayed DQS signal, which is now approximately in quadrature with the received data, is then used as a common sample clock for each of the DQ input receivers for a particular number of bits of data associated with the strobe signal.